What is a quantum dot display (QLED)?

What are quantum dots?

1.1 Concept

Quantum dots are semiconductor nanostructures that bind excitons in three spatial directions. Quantum dots are an important low-dimensional semiconductor material, and their three dimensions are not more than twice the exciton Bohr radius (1-10nm) of the corresponding semiconductor material.

Quantum dots are generally spherical or quasi-spherical, and their diameter is often between 2-20 nm, while the diameter of our hair is about 100,000 nm (100 μm).

1.2 Features

Quantum dots are nano-level semiconductors. By applying a certain electric field or light pressure to this nano-semiconductor material, they will emit light of a specific frequency, and the frequency of the emitted light will change with the size of this semiconductor. Therefore, by adjusting the size of this nano-semiconductor, the color of the light emitted can be controlled. Because this nano-semiconductor has the characteristic of limiting electrons and electron holes (Electron hole), this characteristic is similar to atoms or molecules in nature. , Thus called quantum dots.

Quantum dots are semiconductor nanocrystals. When their particle size is smaller than the Bohr radius of the exciton, the mean free path of electrons is limited to a small range, and it is easy to form exciton pairs with holes. The wave functions of electrons and holes overlap, resulting in an exciton absorption band. The smaller the size of the quantum dot, the greater the probability of forming excitons and the higher the concentration of excitons. This effect is called quantum confinement effect. The quantum confinement effect of quantum dots makes its optical performance different from conventional semiconductor materials. Its energy band structure forms some exciton energy levels near the bottom of the conduction band, resulting in exciton absorption bands, and the recombination of excitons will produce fluorescence radiation. The size of quantum dots is different, the degree to which electrons and holes are quantum confined is different, and their discrete energy level structures are also different.

As the particle size decreases, the confinement degree of electrons and holes increases, leading to an increase in the kinetic energy of the two, that is, an increase in the quantum confinement energy, and the effective band gap of the quantum dot widens, and the corresponding absorption and emission spectra occur Blue shift, and the smaller the size, the greater the blue shift. Therefore, by adjusting the size of the quantum dots, the emission spectrum of the quantum dots can be adjusted.

The energy level of the quantum dot is split due to the quantum confinement effect, and the semiconductor band gap increases as the size of the nanocrystal decreases.

The main properties of quantum dots

1.3 Preparation

1.3.1 Materials

Common quantum dots are composed of IV, II-VI, IV-VI or III-V elements. Specific examples are silicon quantum dots, germanium quantum dots, cadmium sulfide quantum dots, cadmium selenide quantum dots, cadmium telluride quantum dots, zinc selenide quantum dots, lead sulfide quantum dots, lead selenide quantum dots, indium phosphide quantum dots Dots and indium arsenide quantum dots, etc.

Currently used quantum dot materials mainly include cadmium selenide (CdSe) series and indium phosphide (InP) series. The former is mainly used by QD Vision, the latter is mainly used by Nanoco, and Nanosys uses indium phosphide and cadmium hybrid quantum dots. plan. Two kinds of quantum dots have their own advantages and disadvantages. Cadmium selenide is better than high luminous efficiency and wider color gamut. Indium phosphide does not contain cadmium and is not restricted by the EU ROHS standard.

1.3.2 Preparation method

The manufacturing methods of quantum dots can be roughly divided into three categories: chemical solution growth method, epitaxial growth method, and electric field confinement method. These three types of manufacturing methods also correspond to three different types of quantum dots.

Chemical solution growth

In 1993, a research team led by Professor Bawendi of the Massachusetts Institute of Technology synthesized quantum dots of uniform size in an organic solution for the first time. They dissolved three oxygen elements (sulfur, selenium, and tellurium) in tri-n-octyl phosphine oxide, and then reacted with dimethyl cadmium in an organic solution at 200 to 300 degrees Celsius to produce the corresponding quantum dot material (cadmium sulfide). , Cadmium selenide, cadmium telluride). After that, people invented many methods of synthesizing colloidal quantum dots on the basis of this method. Most semiconductor materials can be synthesized by chemical solution growth methods to produce corresponding quantum dots.

Colloidal quantum dots have the advantages of low production cost, high yield, and high luminous efficiency (especially in the visible and ultraviolet bands). But the disadvantage is that the conductivity is extremely low. Since organic ligands are generated on the surface of the quantum dots during the production process, the van der Waals attraction between the quantum dots is offset to maintain its stability in the solution. But this layer of organic ligands greatly hinders the transfer of charges between quantum dots. This greatly reduces the application of nanocrystals in solar cells and other components. Scientists have tried various methods to increase the conductivity of electric charges in this material. Representatively, in 2003, Professor Guyot-Sionnest of the University of Chicago replaced the original long-chain organic ligands with shorter-chain amino compounds, narrowed the quantum dot spacing, and injected a large number of electrons into the quantum dots by electrochemical methods. Inside, the conductivity is increased to 0.01S/cm.

Epitaxial growth

The epitaxial growth method refers to the growth of new crystals on a substrate material. If the crystals are small enough, quantum dots will be formed. According to the different growth mechanism, this method can be subdivided into chemical vapor deposition and molecular beam epitaxy.

The quantum dots grown by this method grow on another type of semiconductor and are easily combined with traditional semiconductor devices. In addition, because there are no organic ligands, the charge transfer efficiency of epitaxial quantum dots is higher than that of colloidal quantum dots, and the energy level is easier to control than colloidal quantum dots. At the same time, it also has the advantages of fewer surface defects. However, since both chemical vapor deposition and molecular beam epitaxy require high vacuum or ultra-high vacuum, the cost of epitaxial quantum dots is higher than that of colloidal quantum dots.

Electric field confinement method

The electric field confinement method refers to the full use of the electric potential of the metal electrode to distort the energy level in the semiconductor to form a constraint on the carriers. Since the required size of quantum dots is at the nanometer level, the metal electrode needs to be fabricated by electron beam exposure. The cost is the highest and the yield is the lowest. However, the quantum dots produced by this method can control their energy level, number of carriers and spin simply by adjusting the gate voltage. Due to the extremely high controllability, such quantum dots are also most suitable for quantum computing.

1.4 Uses of quantum dots

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Application of quantum dot display

2.1 History

In the early 1970s, due to the development of semiconductor epitaxial growth technology, the preparation of nanostructures became possible. First of all, thin-layer two-dimensional nanostructures called Quantum Wells (QW) were synthesized and studied extensively. This nano-thin layer structure is formed by the arrangement of two different semiconductor materials. The electrons and holes are confined in a thin layer of a few nanometers thick, which has obvious confinement effect. By adjusting the composition ratio, the band gap of the quantum well can be changed.

In 2011, Samsung Electronics produced quantum dot light-emitting diodes using organic and inorganic layers as the electron and hole transport layers of the quantum dot light-emitting layer, respectively. By patterning the quantum dot film by the transfer method, Samsung Electronics has produced a 4-inch full-color active matrix QLED display device prototype. Samsung researchers first apply the quantum dot solution on a silicon plate, then evaporate, and then press the protruding part into a quantum dot layer. After removing the surface layer, it is transferred to a glass substrate or a plastic substrate. This process realizes the quantum dot to the substrate. Transfer. Its researchers said that glass substrates or flexible plastic substrates have been used to achieve the production of display prototypes.

Since 2013, quantum dot display technology has been applied to liquid crystal display (LCD) panels. A quantum dot film is assembled between the backlight module and the liquid crystal cell and applied to high-color gamut TVs and tablet computers to achieve a wider range of colors. Domain and lower power consumption.

Sony launched a high-end LCD TV model that uses quantum dot technology in the backlight in June 2013; Amazon also launched a tablet computer that uses quantum dots in the LCD backlight in October 2013.

2.2 Display characteristics of quantum dots

1. High color purity, narrow emission spectrum peak and symmetrical distribution;

2. The emission spectrum is adjustable, and the emission wavelength can be changed by controlling the size and material of the quantum dots, thereby controlling the light-emitting color;

3. Good color performance, covering color gamut greater than 100% NTSC;

4. The luminous efficiency is high, the quantum efficiency is as high as 90%, and the light stability is good;

5. It has the potential to realize nano-level pixels, which can be used to manufacture ultra-high-resolution screens.